The present invention relates to a mechanism for transmitting power from an engine shaft to a power take-off (PTO) shaft.
When operating an agricultural tractor during light load conditions, it is common practice to xe2x80x9cshift up and throttle backxe2x80x9d, in order to improve fuel economy and reduce noise. In modern tractors with electronic controls and power shift transmissions or infinitely variable transmissions, the shift up and throttle back operation is computer controlled to provide optimum performance and full efficiency. However, one cannot xe2x80x9cthrottle backxe2x80x9d for optimum fuel economy when operating the PTO because most PTO driven implements require a constant drive shaft speed to insure proper operation.
A hybrid power transmission system is described in U.S. Pat. No. 5,669,842 wherein PTO shafts can be driven by the engine or by electric motor/generators. However, with this hybrid transmission system, when the clutch is disengaged, the PTO shafts can be driven by motor/generators independently of the engine. Because there is a direct connection between the engine and PTO shafts via the clutch and gears, whenever the clutch is engaged, the speed of the PTO drive shafts will vary as the engine speed varies, and cannot remain constant as the engine speed varies. It would be desirable to provide a mechanism wherein power can be transmitted from an engine to a PTO shaft and wherein the PTO shaft speed can be maintained constant regardless of variations in engine speed.
Some PTO-driven agricultural implements, have metering mechanisms, such as seed drills, planters, potato planters, fertilizer applicators, manure spreaders, and sprayers. Such implements require a PTO speed which is proportional to ground speed. If the tractor wheels slip, the ground speed will slow down, and if the PTO speed remains unchanged, the proportionality of the PTO speed to the ground speed will change, and the speed of the metering mechanism will increase relative to the ground speed. In the case of potato planting, such a condition can cause potatoes to be dumped onto the ground and unevenly planted.
Accordingly, an object of this invention is to provide a transmission which maintains a constant PTO speed despite variations in engine speed.
Another object of this invention is to provide a transmission which is capable of maintaining PTO speed at a desired ratio with respect to ground speed.
These and other objects are achieved by the present invention, wherein a PTO transmission includes a split power transmission unit having an output connected to a PTO shaft, a first input connected to an engine output shaft and a second input connected the output of a variable ratio or speed drive unit. A control unit controls the speed of the variable speed drive unit as a function of sensed engine speed and to maintain the PTO shaft at a constant speed. The split power transmission is preferably a planetary gear unit. The variable speed drive unit preferably includes a variable displacement hydrostatic pump driving a fixed displacement hydrostatic motor which drives a sun gear of the planetary gear unit. This system transmits power to the PTO shaft from both the engine and from the variable speed drive unit. Maintaining a constant PTO shaft speed allows PTO driven implements, such as a grass/hay spreader, to be run optimally. This system also increases fuel economy and, because it reduces engine speed, it reduces noise heard by the operator or others nearby. Also, if the driveline which drives the PTO shaft also drives a hydraulic pump, the hydraulic flow from the pump can be maintained even if the engine is running below its normal speed.
The control unit may also control the speed of the variable speed drive unit as a function of sensed ground speed, sensed output speed, an operator determined speed signal and/or an operator determined ground speed/PTO speed ratio, in order to maintain a desired operator set PTO speed or a desired operator set ground speed/PTO speed ratio. As a result, the PTO speed can be controlled to optimize implement performance without being effected by variations in tractor speed or engine speed.